1. Field of the Invention
This invention relates to reception of a radio signal by using a local oscillation signal which is generated by a phase locked loop (PLL) oscillator with frequency multiplication into a local oscillation frequency and is used in producing an intermediate frequency (IF) signal. More particularly, this invention relates to a radio signal receiving method, to a radio receiver, and to a combination of a local oscillator and intermediate frequency filter circuitry used in a radio receiver for receiving a radio signal to produce an information signal carried by the radio signal.
2. Description of the Related Art
In the manner which will later be described in greater detail, such a radio receiver comprises a high frequency circuit for receiving a radio signal to produce a high frequency signal of a radio frequency, a local oscillator for generating a local oscillation signal of a local oscillation frequency, a mixer or frequency converter for mixing the high frequency signal and the local oscillation signal into an intermediate frequency signal of an intermediate frequency, and receiver circuitry for receiving the intermediate frequency signal to produce an information signal carried by the radio signal. The receiver circuitry comprises intermediate frequency filter circuitry for filtering the intermediate frequency signal into a filtered signal and a demodulator for demodulating the filtered signal into the information signal.
The local oscillator may comprise a phase locked oscillator for generating in a phase locked loop a phase locked oscillation signal of a generated frequency. Connected to the phase locked loop in the local oscillator, a frequency multiplier frequency shifts the phase locked oscillation signal into the local oscillation signal with the generated frequency shifted by a multiplication factor into the local oscillation frequency.
In the radio receiver of the type described, a carrier to noise (C/N) ratio depends on a noise component near the generated frequency. If a spurious signal or a spurious electromagnetic wave coexists with the radio signal, the carrier to noise ratio adversely influences a sensitivity of the radio receiver to a great extent. More specifically, the information signal becomes unclear due to noise when the carrier to noise ratio is small. The carrier to noise ratio depends, in turn, on the multiplication factor. It is known that the carrier to noise ratio is degraded in general by an amount proportional in decibels (dB) to an expression as follows: EQU 20 log n,
where n represents the multiplication factor.
It is therefore understood that the generated frequency should be as high as possible to reduce the multiplication factor. This, however, necessitates a high operation frequency of a prescaler included in the phase locked loop. When put in operation at a high frequency, the prescaler must be supplied with a high voltage. This results in an undesirable increase in power consumption of the radio receiver.